Field of the Invention
This invention relates generally to identifying ground faults in a stator winding of an electrical generator and, more particularly, to a device and method for detecting and analyzing neutral ground faults which includes measuring a direct current component of a neutral ground current in a generator stator grounding conductor, analyzing the direct current flow and comparing it with other ground fault indicators, and providing diagnosis and recommended actions based on the analysis.
Description of the Related Art
Electrical power generators convert mechanical power into one or more electrical currents, one from each series of coils, or winding. It is beneficial to protect electrical power generators against external faults and internal faults. Generators can be protected against external faults by several circuit breakers that isolate the faults that could occur in the power plant network (e.g., transformers, buses, lines . . . ). At the same time, electrical power generators can also be protected against faults that could occur inside the machine itself.
Many electrical power generators in power plants are of a three-phase design that utilizes a “Y” winding configuration. In this type of design, all three phases of the stator are connected at one end, typically by a large bus bar, which corresponds to the center of the “Y”. Usually, this bus bar is also grounded. In most instances the grounding point that is selected is the power plant's earth ground and the bus bar is connected to the ground through a transformer that is commonly referred to as a neutral grounding transformer.
In an ideal operation, the current flow to ground would hypothetically be nearly zero. A very small current will flow to ground through the generator side of the neutral grounding transformer, largely due to capacitance of the complete generator, and all components tied to it electrically. When a fault to ground occurs in any of these electrically connected components, the current will rise. The transformer is typically designed to limit ground fault current to 20 Amperes to minimize consequential damage from the ground fault.
For example, a ground fault in a generator stator winding is one of the most frequent types of internal electrical generator fault. A ground fault can be caused by, for example, physical damage to the stator winding or aging of the insulation of the stator and can cause additional damage to the stator and cause the electrical power generator to fail.
In the past, analysis of the current passing through a neutral grounding circuit (e.g., that includes a neutral grounding transformer and an impedance) has focused on the alternating current component which is naturally present and dominant. However, a direct current component may also be present in a generator grounding circuit, and this direct current component cannot be detected in the output of the neutral grounding transformer. It would be desirable to detect the direct current component in the grounding circuit, as the direct current component may be useful in diagnosing fault conditions in the generator.